After buying a 2nd hand Snorkel from Noel last weekend, I obviously had a bit of a dilemma as there was no installation template, and the pipe extruding from the 'foot' of the snorkel makes it impossible to offer it up close enough to the panel to mark any holes.

Then I did the following:

Paper Mache snorkel impression with and curvature (and pillar holes - not visible on the pic)